1. Field of the Invention
This invention relates to novel aromatic amine terminated bisaspartimides which are useful as precursors for preparing novel, cross-linked, high temperature stable polymeric resins, and to a method for making the bisaspartimides. More particularly, the invention relates to novel 4,4'-bis{N.sup.2 -[4-(4-aminophenoxy)phenyl]aspartimido}diphenylmethane, which may be thermally polymerized to produce a tough polymer. If further relates to a process for preparing these bisaspartimide compounds by a Michael-type reaction of an aromatic bismaleimide and an aromatic diamine in an aprotic solvent. It also relates to novel cloth laminates incorporating the polymers of this invention.
2. Description of the Prior Art
High temperature resistant polymers are used extensively in advanced aerospace structures in which structural integrity must be retained during continuous use at temperatures of 325 degrees C. and above. The stringent requirements of space technology and of other industrial applications for thermal protective materials have led to the development of several classes of heat- and flame-resistant heterocyclic polymers. Aromatic polyimides, developed during the 1960s, have met these requirements to a large extent. They are prepared by a condensation reaction. Loss of desirable mechanical properties and problems of reproducibility have been observed because of voids created by the elimination of water during their formation in situ, difficulty in the removal of high boiling point solvents, or both. As a result, the use of these polyimides as laminating resins or for adhesives has been limited. Such polyimides are disclosed, for example, in the following references: C. E. Sroog, J. Polym. Sci., 16, 1191 (1967); M. L. Wallach, J. Polym. Sci. A-2, 5, 653 (1967); R. Ikeda, J. Polym. Sci., B, 4, 353 (1966); C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Bear, W. M. Edwards, and K. L. Oliver, J. Polym. Sci. A, 3, 1373 (1965); R. A. Dine-Hart and W. W. Wright, J. Appl. Polym. Sci., 11, 609 (1967).
Initial attempts to overcome these processability problems led to the development of addition polyimides based on short, preimidized segments which polymerize thermally through end groups without loss of volatiles, such as disclosed by F. Grundschober and J. Sambeth, U.S. Pat. Nos. 3,380,964; 3,523,996; French Pat. No. 1,455,514; Chem. Abstr., 65, 10746 (1966). However, these addition polyimides were found to be inherently brittle because of extensive cross-linking that occurs during polymerization.
P. Kovacic, U.S. Pat. No. 2,818,405; Chem. Abstr. 52, 5018e (1958), reported the reaction of amine capped liquid elastomeric prepolymers with bismaleimides to afford polymers. However, these elastomer based polymers were not heat resistant. M. Bargain et al, U.S. Pat. No. 3,562,223; Chem. Abstr. 72, 4047 (1970) disclose cross-linked resins obtained by the melt polymerization of bismaleimide and the addition of diamine to the melt in ratios of 1.2:1 to 50:1 with further polymerization. The prepolymers thus formed are used to prepare prepregs via their N-methylpyrrolidone solution to give laminates. However, these resins show a tendency to polymerize in solution, significantly increasing the solution viscosity and causing processing difficulties.
A wide variety of polyimide resins derived from bismaleimides and/or aromatic diamines, in combination with various other materials, are known in the art. Such resins are disclosed in, for example, the following U.S. Pat. Nos. 3,671,490; 3,740,378; 3,766,138; 3,855,239; 3,897,393; 4,020,303; 4,060,515; 4,075,171; 4,116,937; 4,280,946; 4,283,521; 4,316,843; 4,273,916; 4,346,206. See also J. V. Crivello, J. Polym. Sci., 11, 1185 (1973).
While the art pertaining to high temperature stable resins and precursors is a well developed one, there is a continuing need for a polyimide binder that can be used as a matrix for low void fiber reinforced composites that retain favorable properties and have thermooxidative stability. Such binders should also be capable of being used in rapid, high pressure, matched-die molding, as well as in lower pressure vacuum bag autoclave molding. It has not been found to be desirable to use a high molecular weight polymer in a solution as a binder because the solution is too viscous at a high solids content for handling. At low solids concentration, there is usually insufficient resin pick up in one pass in the prepregging operation.
Whereas other mixed bismaleimide-aromatic amine systems polymerize thermally by reaction of vinyl groups with either other vinyl groups or with free amino groups, depending on the stoichiometry of the system, the herein described materials, having all or substantially all their vinyl group previously prereacted with one amino group of an aromatic diamine, is believed to cure by an imide ring opening reaction to form amides. It is also known that an imide ring opening reaction occurs as a side reaction of an amino acid (proline) reacting with a substituted ester, as reported by J. Savrda, J. Org. Chem., 42, 3199 (1977).